Every year, millions of people don’t file their taxes. It might be because they’re afraid they owe money to the IRS that they can’t pay, or because they think their income falls below the threshold required to file.

But if you didn’t file taxes in 2014, you might want to pay attention.

According to the IRS, Americans left $1.1 billion in unclaimed federal income tax refunds in 2014. The money belongs to about one million taxpayers who didn’t file their returns, and the IRS says about half of the refunds are for more than $847.

The IRS is making this announcement now because time is running out for people to claim their refunds. Generally, Americans have three years to file a past return before their refund money becomes property of the U.S. Treasury. That means April 17, 2018, is the last day for people who didn’t file their returns in 2014.

Failing to file federal returns also varies greatly by state. In Vermont, 27,000 people failed to file their return in 2014, leaving behind about $2 million. In Texas, 108,000 people didn’t file a tax return in 2014, leaving behind a whopping $122 million.

If this notice applies to you, there a few things to consider. First, if you file your 2014 return, your refund will be held if you also neglected to file a return in 2015 and 2016. Also, if you owe money for child support or past due debts (like student loans) the IRS will take those fees out of your refund.

Filing a past return could also be lucrative. The Earned Income Tax Credit (EITC) is designed to help low- and moderate-income families. In 2014 the tax credit was worth as much as $6,143, which you could claim if your income was below a certain threshold. For example, a single person with three or more children could qualify for the EITC if they made less than $46,997.

Those wanting to file an old tax return can find the appropriate forms on the IRS documents page. To get your hands on old income forms (W2, 1098 or 1099), taxpayers can request copies from their employer or bank. If unable to do so, you can also order a free wage and income transcript at IRS.gov using the Get Transcript Online tool.

The window to file your 2014 taxes ends April 17, 2018, so all returns must be mailed and postmarked by that date.